I'm currently learning ARM64 assembly and I'm having trouble understanding how bitmask immediates are implemented and validated. I've read that ARM64 has specific rules for what constitutes a valid bitmask immediate, but I'm finding it difficult to grasp these rules in practice.

https://developer.arm.com/documentation/dui0802/a/A64-General-Instructions/AND--immediate-?lang=en

Is the bitmask immediate. Such an immediate is a 32-bit or 64-bit pattern viewed as a vector of identical elements of size e = 2, 4, 8, 16, 32, or 64 bits. Each element contains the same sub-pattern: a single run of 1 to e-1 non-zero bits, rotated by 0 to e-1 bits. This mechanism can generate 5,334 unique 64-bit patterns (as 2,667 pairs of pattern and their bitwise inverse). Because the all-zeros and all-ones values cannot be described in this way, the assembler generates an error message.

For instance, I understand that 0x00FF00FF is valid:

AND x0, x1, #0x00FF00FF  // Valid

For example, I tried using the following instruction:

AND x0, x1, #0xF0F0

However, I get the following error:

<source>:15: Error: immediate out of range at operand 3 -- \\\`and x0,x1,#0xF0F0'

1. What are the exact rules for a valid bitmask immediate in ARM64?
2. Why is 0xF0F0 considered an invalid bitmask immediate?
3. How can I determine if a given bitmask is valid or not?
4. Please help me understand this subject. I have read a lot of sources and asked people questions but I can't understand it.